CLOUDSTACK-7854: Don't have CentOS 5.3 built-in template when we've 5.6

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>
(cherry picked from commit 318f7159a0f1456e1058e6dee43887b4683310de)
Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>